Concentration and Reversible reactions

Recap: Equilibrium

  • Happens in a sealed container (no substances escape).

  • At equilibrium, forward and reverse reactions occur at the same rate.

Le Chatelier’s Principle

If a system is at equilibrium and the conditions change, the system responds to counteract the change.

  • "System" simply means "the reaction."

Example Reaction

2NO2 (g)⇌N2O4 (g)2NO_2\,(g)\rightleftharpoons N_2O_4\,(g)

  • Forward:2NO22NO_2 ​ reacts to formN2O4N_2O_4 ​.

  • Reverse: N2O4N_2O_4 breaks down to form NO2NO_2 .

Changes in Concentration

  1. Increase concentration of NO₂ (reactant):

    • System opposes change → more N₂O₄ is formed.

  2. Decrease concentration of N₂O₄ (product):

    • System opposes change → more NO₂ reacts to make N₂O₄.

  3. Increase concentration of N₂O₄ (product):

    • System opposes change → more N₂O₄ breaks down into NO₂.

✅ Key Idea:
When concentration of a reactant/product changes, the equilibrium shifts to use up what has been added or replace what has been removed.